I've changed my phone number and forget my iCloud what should I do?

Note : You need account recovery from the server as Apple ID is known but password is forgotten  How to use account recovery when you can’t reset your Apple ID password - Apple Support

The Apple ID say example1@icloud.com exists on server and you know the password and its account credentials , visit your cellular service centre with personal identification proofs .

If the account is set with two factor authentication from iPhone .

Issue a sim on behalf of the same old number and insert the sim Set up cellular service on iPhone - Apple Support

In a new iPhone that is purchased Turn on and set up iPhone - Apple Support

The Apple ID and password is active so is the 2FA active on the iCloud server , once a new trusted number is added the code will start appearing on the new trusted number , and the person can access the account .

See the article Get a verification code and sign in with two-factor authentication - Apple Support

Then access the account and remove an old verified number from the account .

Go to your Apple ID account page and enter your Apple ID and password.

In the Security section, click Edit.

Click Add a Trusted Phone Number. 

To add a number, enter the phone number and verify it with a text. 

To remove a number, click cross sign next to the phone number you want to remove.